Size and weight is a major design variable in the design of any power supply. The size and weight of a conventional pulse- width-modulated (PWM) supply, of a given power rating, may be significantly reduced by increasing its switching frequency. This reduces the size of energy storage components in the circuit since energy storage levels per cycle are reduced at the higher frequency. Without a change in circuit design the increase in switching frequency increases power losses due to the increased number of power switching transitions within a given time interval. Therefore, in order to operate power supplies at high switching frequencies, and gain the full benefit of reduced size and weight, the power switching losses need to be minimized. A number of soft-switching power processing techniques, using "soft" waveforms have been developed to reduce these switching losses. These switching losses are reduced, in some power processing techniques, by circulating energy due to power switch transitions within the power supply as opposed to allowing it to dissipate.
One of the first power processing techniques developed to minimize switching transition losses was the zero-voltage-switched (ZVS) quasi-resonant-converters (QRC). The voltage across the switch is constrained to be zero at the off-to-on transition of the power switch by use of resonant signals. In quasi-resonant-converters resonant signals are quenched within a single cycle of operation. The ZVS-QRC significantly reduces the turn-on switching loss without increasing the current stress through the converter (relative to conventional PWM converters). Unfortunately, in single-ended topologies the voltage stress applied to the principal active power switch is increased considerably and is dependent on the line and load voltage. Furthermore, the junction capacitance of the rectifier diodes causes large parasitic ringing. Parasitic ringing is detrimental to the operation of the converter and this ringing in conjunction with the increased voltage stress of the active switch, limits the number of applications of ZVS-QRCs.
Zero-voltage-switched (ZVS) multi-resonant-converters (MRC), developed subsequently, minimize the effect of this parasitic ringing present in ZVS-QRCs. ZVS-MRCs absorb all of the major parasitic elements in a typical power supply. But soft-switching operation of the semiconductor components in a ZVS-MRC is achieved at the expense of an increased circulation of energy. ZVS-MRCs operate with increased current and voltage stresses as compared to traditional PWM converters, resulting in increased conduction losses that limit application of ZVS-MRCs to high-frequency, low power applications.
A recent family of switching converters utilizes zero voltage/zero- current transition (ZV/ZC-T) in which switching losses are significantly reduced in both active and passive power switches. These switching converters, as well as those converters of the prior art described above, generally have a limited load range because of the requirement of zero voltage switching. It is desirable for switching converters to have a wide range of load accommodation in order to achieve commercial acceptability.